This invention relates generally to oximeters that measure arterial blood oxygen saturation (SaO2) levels in tissues. More specifically, this invention relates to oximeters that use the pulsatile component of light of multiple wavelengths to determine the amount of arterial blood oxygen saturation.
The arterial blood oxygen saturation and pulse rate of an individual are of interest for a variety of reasons. For example, emergency or surgical care settings can use information regarding oxygen saturation to signal changing physiological factors, the malfunction of anesthesia equipment, or physician error. Similarly, in the intensive care unit, oxygen saturation information can be used to confirm the provision of proper patient ventilation or to optimize a gradual reduction and eventual removal from assisted ventilation.
The proper utilization of many lifesaving medical techniques and treatments depends upon the attending physician continually obtaining accurate and up-to-date information regarding various bodily functions of the patient. Heart rate, blood pressure, and arterial oxygen saturation are among the most critical information that a physician needs to determine an optimal course of treatment. Continuous provision of this information is crucial to allow the physician to immediately adopt a procedural course that will best meet a patient""s needs.
Arterial oxygen saturation (SaO2) is expressed as a percentage ratio of hemoglobin which is bound to oxygen (i.e., oxygenated hemoglobin (HbO2 or xe2x80x9coxyhemoglobinxe2x80x9d)) to the total hemoglobin in the patient""s blood (including both oxygenated (HbO2) and non-oxygenated hemoglobin (Hb)), as represented by the following equation:
SaO2=([HbO2]/([Hb]+[HbO2]))xc3x97100%
In a healthy patient, the SaO2 value is generally above 95% since blood traveling through the arteries has just passed through the lungs and has been oxygenated. As blood courses through the capillaries, however, oxygen is off-loaded into the tissues and carbon dioxide is on-loaded into the hemoglobin. Thus, the oxygen saturation levels in the capillaries (ScO2) is always lower than in the arteries. Once the blood has provided oxygen to the body tissue, the blood returns to the heart through the veins. Accordingly, the blood oxygen saturation levels in the veins is even lower still (i.e., about 75%).
Importantly, if the patient""s arterial oxygen saturation level is too high or too low, the physician can take corrective action, such as reducing or increasing the amount of oxygen being administered to the patient, only after he or she learns of the incorrect saturation level. Proper management of arterial oxygen saturation is particularly important in neonates where SaO2 must be maintained high enough to support cell metabolism but low enough to avoid damaging oxygen-sensitive cells in the eye, which could cause impairment or complete loss of vision. Accordingly, in many applications, the continual provision of up-to-date information regarding the patient""s pulse rate and oxygen saturation level is crucial to allow the physician to detect harmful physiological conditions before any observable physical manifestations of a substantial risk of injury arise. In settings such as operating rooms and in intensive care units, monitoring and recording these indicators of bodily functions is particularly important. For example, when an anesthetized patient undergoes surgery, it is generally the anesthesiologist""s role to monitor the general condition of the patient while the surgeon proceeds with his tasks.
Typical techniques for measuring these characteristics include invasive procedures, such as using an inserted catheter to measure blood pressure and to extract periodic blood samples, or non-invasive techniques. Unfortunately, although invasive procedures are typically more accurate than non-invasive ones, they generally take several minutes to obtain results. These wasted minutes can be crucial in many medical situations as human tissue can begin to degenerate with lack of sufficient oxygen in just a few minutes. Non-invasive techniques are therefore generally preferred, not only because they avoid the painful insertion of needles or other instrumentation into a patient""s body, but also because they offer a quicker response to changing physiological characteristics of the patient. Noninvasive techniques are also desirable when complex blood diagnostic equipment is not available, such as, for example, when a home health care provider performs a routine check-up in a patient""s home.
The term xe2x80x9coximetryxe2x80x9d has been adopted in the art to refer to noninvasive apparatus and methods for determining blood oxygen saturation levels. Conventional types of oximeters include finger oximeters, earlobe oximeters, and fetal oximeters. Conventional oximetry systems make use of the fact that the absorption characteristics of different blood components, namely, HbO2 and Hb, differ depending on which wavelength of light (e.g., infrared or visible portions of the spectrum) is being used. Accordingly, typical noninvasive oximetric systems impinge at least both visible and infrared light upon a body part, such as a finger, and then estimate the SaO2 level using the relative proportions of visible and infrared light transmitted through or reflected by the body tissue. Undesirably, however, these conventional systems inherently include some inaccuracy, which increases to a substantial error for low (50-70%) SaO2 levels, due to, among other things, the inclusion of capillary blood as well as arterial blood in the light measurement readings.
In an effort to improve the accuracy of the SaO2 values obtained using two wavelengths of light, some systems have utilized the pulsatile component of the transmitted or reflected light beam to distinguish variations in the detected intensity of the light beam which are due to changes in blood components from other causes. This approach is generally referred to as pulse oximetry. Using the pulsatile signal modulating the light beams for obtaining an SaO2 estimate provides a significant improvement in accuracy over non-pulse oximetry systems.
xe2x80x9cPulsed oximetersxe2x80x9d are therefore oximeters which measure the arterial component of the blood perfusion, to yield the arterial oxygen saturation (SaO2) level, using the pulsatile component of a light transmission signal. Companies have built special circuitry and developed algorithms to obtain good signal-to-noise ratios for this pulsatile measurement. These conventional circuits and algorithms typically yield a pulsatile factor (R) which is based in part on the ratio of the pulsatile component of light measurements at a red wavelength (eg., 600-800 nm) and at an infrared wavelength (eg., 800-1000 nm). More specifically, the pulsatile factor R is equal to a ratio of the pulsatile component divided by the steady-state component of light at the red wavelength to the pulsatile component divided by the steady-state component of light at the infrared wavelength, as shown by the equation:
R=(AC/DC)red/(AC/DC)infrared
The pulsatile factor R therefore properly corrects for variation in the power of the light sources and photodetectors comprising the measurement device. As will be discussed later, the ratio R does not, however, correct for the background tissue optics consisting of tissue thickness, tissue blood perfusion, light scattering, and boundary conditions such as bones and the air/tissue surface.
As explained above, traditional oximeters calibrate their pulsatile factor R measurement using non-invasive light transmission or reflection analysis as opposed to direct measurement of arterial SaO2 measured with catheters inserted in arterial vessels. More specifically, pulse oximeters monitor blood oxygen content by measuring the absorption of light in an arterialized vascular bed. Since oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) absorb light differently, the relative concentration of each blood component and thus the SaO2 can be determined by measuring absorbed light at two different wavelengths. Pulse oximetry is now an established standard of care during anesthesia and in neonatal and adult critical care.
The basic design of conventional pulse oximeter probes includes both red and infrared light emitting diodes (LEDs) and a photodetector (or light transducer). These components are arranged so that the LEDs illuminate a particular section of arterialized tissue. The detector collects the light from the LEDs which has been transmitted through the tissue section but not absorbed by the skin, bone, blood and other physiologic absorbers. The steady-state (DC) and time-varying (AC) components of this signal are then used to calculate the fraction of the arterial blood which is oxygenated.
Pulse transmittance oximetry basically involves measurement of how the arterial blood in body tissue affects the intensity of light passing therethrough. More particularly, the volume of blood in the tissue is a function of the arterial pulse, with a greater volume present at systole and a lesser volume present at diastole. Because blood absorbs some of the light passing through the tissue, the intensity of the light emerging from the tissue is inversely proportional to the volume of blood in the tissue. Thus, the emergent light intensity will vary with the arterial pulse and can be used to indicate a patient""s pulse rate. In addition, the absorption coefficient of HbO2 is different from that of Hb for most wavelengths of light. For that reason, differences in the amount of light absorbed by the blood at two different wavelengths can be used to indicate the level of arterial oxygen saturation, SaO2. Thus, by measuring the amount of light transmitted through an earlobe or finger, for example, a pulse oximeter can be used to determine both the patient""s pulse rate and arterial blood oxygen saturation.
The intensity of light transmitted through an earlobe, finger, or other body part is a function of the absorption coefficient of both xe2x80x9cfixedxe2x80x9d and xe2x80x9cvariablexe2x80x9d components. Examples of xe2x80x9cfixedxe2x80x9d components include bone, tissue, skin, and hair. Examples of xe2x80x9cvariablexe2x80x9d components include the volume of blood in the tissue. The intensity of light transmitted through the tissue is generally expressed as a function of time. It includes a baseline (or xe2x80x9cDCxe2x80x9d) component, which varies slowly with time and represents the effect of the fixed components on the light transmission. It further includes a periodic pulsatile (or xe2x80x9cACxe2x80x9d) component, which varies more rapidly with time and represents the effect that changing tissue blood volume has on the light. Because the attenuation produced by the fixed tissue components does not contain information about pulse rate and arterial oxygen saturation, the pulsatile signal is of primary interest. In that regard, many of the transmittance oximetry techniques of the prior art eliminate the baseline component from the signal analyzed.
For example, U.S. Pat. No. 2,706,927 (Wood) measures light absorption at two avelengths under a xe2x80x9cbloodlessxe2x80x9d condition and a xe2x80x9cnormalxe2x80x9d condition. In the bloodless condition, as much blood as possible is squeezed from the tissue being analyzed. Then, light at both wavelengths is transmitted through the tissue and absorption measurements made. These measurements indicate the effect that all non-blood tissue components have on the transmission of light through the tissue. When normal blood flow has been restored to the tissue, a second set of measurements is made that indicates the influence of both blood and non-blood components. The difference in light transmission measurements between the two conditions is then used to determine the average oxygen saturation of the tissue, including the effects of both arterial and venous blood. This process essentially eliminates the DC, non-blood component from the signal used to determine oxygen saturation.
For a number of reasons, however, the Wood method fails to provide the necessary accuracy. For example, a true bloodless condition cannot be obtained practically. In addition, efforts to obtain a bloodless condition, such as by squeezing the tissue, may result in a different light transmission path for the two conditions. In addition to problems with accuracy, the Wood approach is both inconvenient and time consuming and can cause damage to the tissue.
A more refined approach to pulse transmittance oximetry is disclosed in U.S. Pat. No. 4,086,915 (Kofsky et al.). The Kofsky et al. patent is of interest for two reasons. First, the technique of Kofsky et al. automatically eliminates the effect that fixed components in the tissue have on the light transmitted therethrough, avoiding the need to produce bloodless tissue. More particularly, as developed from the Beer-Lambert law of absorption for a clear medium with no light scattering, the derivatives of the intensity of the light transmitted through the tissue at two different wavelengths, when multiplied by predetermined pseudo-coefficients, can be used to determine oxygen saturation. Basic mathematics indicates that such derivatives are substantially independent of the DC component of the intensity, however the simple math does not hold for an optically turbid medium such as tissue with strong light scattering. The pseudo-coefficients are determined through measurements taken during a calibration procedure in which a patient first respires air having a normal oxygen content and, later, respires air of a reduced oxygen content. Unfortunately, this process is cumbersome.
Another reference addressed to pulse transmittance oximetry is U.S. Pat. No. 4,407,290 (Wilber). According to Wilber, light pulses produced by LEDs at two different wavelengths are applied to a body part, such as an earlobe. A sensor responds to the light transmitted through the earlobe, producing a signal for each wavelength having a DC and AC component resulting from the presence of constant and pulsatile absorptive components, respectively, in the earlobe. A normalization circuit employs feedback to scale both signals so that the DC, non-pulsatile components of each are equal and so that these offset voltages can be removed. Decoders separate the two signals, so controlled, into channels A and B where the DC component is removed from each. The remaining AC components of the signals are amplified and combined in a multiplexer prior to analog-to-digital (A/D) conversion. Oxygen saturation is then determined by a digital processor.
European Patent Application No. 83,304,939.8 (New, Jr. et al.) discloses yet another pulse transmittance oximeter. According to New, Jr. et al., two LEDs expose a body member, such as a finger, to light having red and infrared wavelengths, with each LED having a one-in-four duty cycle. A detector produces a signal in response to the light that is split into two channels. The one-in-four duty cycle allows negatively amplified noise signals to be integrated with positively amplified signals including the detector response and noise, thereby eliminating the effect of noise on the signal produced. The resultant signals include a substantially constant DC component and an AC component. To improve the accuracy of a subsequent analog-to-digital (A/D) conversion, a fixed DC value is subtracted from the signal prior to the conversion. This level is then added back in by a microprocessor after the conversion. Logarithmic analysis is avoided by the microprocessor because for each wavelength of light transmitted through the finger, a quotient of the AC component over the constant DC component is determined. The ratio of the two quotients is then determined and fitted to a curve of independently derived oxygen saturation levels. To compensate for the different transmission characteristics of different patient""s fingers, an adjustable drive source for the LEDs is provided.
Pulsed oximetry has been successful as a trend detector to detect a sudden fall in SaO2 from the normal value of approx. 95%. However, pulsed oximetery has failed to prove accurate over a broad range of saturation levels. The subject-to-subject and tissue site-to-site variation in tissue blood perfusion is too great to allows a single calibration curve to relate R to SaO2 for all cases. Pulsed oximetry needs adaptive calibration to properly interpret R values based on pulsatile light transmission to yield accurate SaO2 values. There is a significant need for an oximeter with adaptive calibration.
One object of the present invention is to enable a method of determining an arterial blood oxygen saturation level that is accurate over various oxygen saturation levels.
Another object of the present invention is to enable a method of determining an arterial blood oxygen saturation level that is adaptively calibrated to give accurate blood oxygen saturation values over a range of oxygen saturation levels.
The present invention is an oximetry system that uses a plurality of sets of calibration curves, each containing a plurality of calibration curves, to permit accurate calibration of the system over a range of oxygen saturation levels.
According to the invention, an adaptively calibrated pulse oximeter uses the steady-state component of light transmission measurements to select a proper calibration curve. The selected calibration curve is then used to properly interpret the pulsatile factor obtained from the conventional measurement of the pulsatile component of the light signals to yield an accurate arterial blood oxygen saturation level determination.
In general, one method of determining an arterial oxygen saturation level according to this invention proceeds by using the DC components of light transmission measurements for both red and infrared light to determine a blood volume fraction/mixed blood oxygen saturation value pair. The unique pair is then used to select an appropriate calibration curve. Once the appropriate calibration curve has been selected, a pulsatile actor can then be used to determine the corresponding arterial blood oxygen saturation value.
More specifically, light measurements at the red and infrared wavelengths are taken with the probe in air (or some other standard medium). These calibration measurements are one-time measurements to allow correction for variation in the power of the light source and for variation in the responsivity of the detector. Subsequent measurements of the tissue are normalized by the calibration measurements. The normalized measurements are then used to determine the volume fraction of blood in the tissue and the mixed blood oxygen saturation value from a grid mapping. An appropriate calibration curve from among a plurality of calibration curves can then be selected using the blood volume fraction and the mixed blood oxygen saturation value.
According to one embodiment, a plurality of sets of calibration curves can also be provided. Each set of calibration curves includes a plurality of calibration curves relating the pulsatile factor to the arterial blood oxygen saturation level. The volume blood fraction and the mixed blood oxygen saturation value are used to select an appropriate calibration curve from among the plurality of calibration curves. Using the appropriate calibration curve, the pulsatile factor can be properly interpreted to yield an arterial blood oxygen saturation value. If the blood volume fraction or mixed blood oxygen saturation value of the patient""s tissue site changes, the calibration will change accordingly to adaptively calibrate the determination of arterial blood oxygen saturation values.